1. Field of the Invention
The present invention concerns a method to generate a fat-reduced, spatially resolved magnetic resonance spectrum of an examination subject and a magnetic resonance system for implementing such a method.
2. Description of the Prior Art
The modality of spatially resolved MR spectroscopy differs from the modality of MR imaging in that, in addition to the spatial resolution of the MR signals, the chemical shift should also be spatially resolved in spectroscopy. (“Spatial resolution” and “spatially resolved” mean assigning values to the signal in question for associated positions in space.) In general, spectroscopic MR methods must in particular account for the circumstances that the metabolic products of interest exhibit a concentration that is approximately four orders of magnitude lower than the water and fat molecules shown in MR imaging. The significant information of spatially resolved MR spectroscopy lies in the spectral range between the line of free water at 0 ppm and the lines of the fat protons at approximately 3 ppm. For example, these lipid signals appear outside of the examined region or VOI (volume of interest) in the brain spectrum of the region and make the interpretation and quantification of the lines of metabolites (such as NAA, choline, GABA and inositol) more difficult. The method designated as the CSI (Chemical Shift Imaging) method is known in spectroscopic imaging. In CSI mammo-spectroscopies, as well as in prostate spectroscopies, the determination of the citrate/choline/creatine ratio can be very strongly distorted by the contamination of the dominating fat spectrum, assuming it is not made impossible in the first place.
In the prior art it is known to suppress the contribution of the fat spectrum by the application of pulses that excite the fat signals outside of the examined range and destroy their magnetization, so that these fat signals make no or only a slight signal contribution in the subsequent imaging spectroscopy. However, in many applications these saturation bands that are to be spatially placed can only be insufficiently adapted to the anatomy. The magnetization of the signal is additionally restored again in part by the refocusing effect of the many pulses on the magnetization in the overlap range of the saturation pulses, so artifacts can arise. Furthermore, the placement of the many different saturation bands to define the saturated regions is time-consuming.